Past

Cell locomotion is essential for early development, angiogenesis, tissue regeneration, the immune response, and wound healing in multicellular organisms, and plays a very deleterious role in cancer metastasis in humans. Locomotion involves the detection and transduction of extracellular chemical and mechanical signals, integration of the signals into an intracellular signal, and the spatio-temporal control of the intracellular biochemical and mechanical responses that lead to force generation, morphological changes and directed movement. While many single-celled organisms use flagella or cilia to swim, there are two basic modes of movement used by eukaryotic cells that lack such structures -- mesenchymal and amoeboid. The former, which can be characterized as `crawling' in fibroblasts or `gliding' in keratocytes, involves the extension of finger-like filopodia or pseudopodia and/or broad flat lamellipodia, whose protrusion is driven by actin polymerization at the leading edge. This mode dominates in cells such as fibroblasts when moving on a 2D substrate. In the amoeboid mode, which does not rely on strong adhesion, cells are more rounded and employ shape changes to move -- in effect 'jostling through the crowd' or `swimming'. Here force generation relies more heavily on actin bundles and on the control of myosin contractility. Leukocytes use this mode for movement through the extracellular matrix in the absence of adhesion sites, as does Dictyostelium discoideum when cells sort in the slug. However, recent experiments have shown that numerous cell types display enormous plasticity in locomotion in that they sense the mechanical properties of their environment and adjust the balance between the modes accordingly by altering the balance between parallel signal transduction pathways. Thus pure crawling and pure swimming are the extremes on a continuum of locomotion strategies, but many cells can sense their environment and use the most efficient strategy in a given context. We will discuss some of the mathematical and computational challenges that this diversity poses.

I shall discuss recent work in which bounds are obtained, generalising/specialising earlier work for general linear groups

The induced graph removal lemma states that for any fixed graph $H$ on $h$ vertices and any $e\textgreater 0$ there exists $d\textgreater0$ such that any graph $G$ with at most $d n^h$ induced copies of $H$ may be made $H$-free by adding or removing atmost $e n^2$ edges. This fact was originally proven by Alon, Fischer, Krivelevich and Szegedy. In this talk, we discuss a new proof and itsrelation to various regularity lemmas. This is joint work with Jacob Fox.

We discuss conjectures and results concerning small gaps between primes. In particular, we consider the work of Goldston, Pintz and Yildrim which shows that infinitely often there are gaps which have size an arbitrarily small proportion of the average gap.

The Farrell-Jones Conjecture predicts a homological formula for K-and L-theory of group rings. Through surgery theory it is important for the classification of manifolds and in particular the Borel conjecture. In this talk I will give an introduction to this conjecture and give an overview about positive results and open questions.

The vacant set is the set of vertices not visited by a random walk on a graph G before a given time T. In the talk, I will discuss properties of this random subset of the graph, the phase transition conjectured in its connectivity properties (in the `thermodynamic limit'

when the graph grows), and the relation of the problem to the random interlacement percolation.  I will then concentrate on the case when G is a large-girth expander or a random regular graph, where the conjectured phase transition (and much more) can be proved.

Khovanov homology is an invariant of knots in S^3 which categorifies the Jones polynomial. Let C be a singular plane curve. I'll describe some conjectures relating the geometry of the Hilbert scheme of points on C to a variant of Khovanov homology which categorifies the HOMFLY-PT polynomial. These conjectures suggest a relation between HOMFLY-PT homology of torus knots and the representation theory of the rational Cherednik algebra. As a consequence, we get some easily testable predictions about the Khovanov homology of torus knots.

The study of superconformal Chern-Simons theories has led to a deeper understanding of M-theory and a new example of the AdS/CFT correspondence. In this talk, I will give an overview of superconformal Chern-Simons theories and their gravity duals. I will also describe some recent work on scattering amplitudes in these theories.

Liquid impact is a key issue in various industrial applications (seawalls, offshore structures, breakwaters, sloshing in tanks of liquefied natural gas vessels, wave energy converters, offshore wind turbines, etc). Numerical simulations dealing with these applications have been performed by many groups, using various types of numerical methods. In terms of the numerical results, the outcome is often impressive, but the question remains of how relevant these results are when it comes to determining impact pressures. The numerical models are too simplified to reproduce the high variability of the measured pressures. In fact, for the time being, it is not possible to simulate accurately both global and local effects. Unfortunately it appears that local effects predominate over global effects when the behaviour of pressures is considered.

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Having said this, it is important to point out that numerical studies can be quite useful to perform sensitivity analyses in idealized conditions such as a liquid mass falling under gravity on top of a horizontal wall and then spreading along the lateral sides. Simple analytical models inspired by numerical results on idealized problems can also be useful to predict trends.

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The talk is organized as follows: After an introduction on some of the industrial applications, it will be explained to what extent numerical studies can be used to improve our understanding of impact pressures. Results on a liquid mass hitting a wall obtained by various numerical codes will be shown.

A class of stochastic individual-based models, written in terms of coupled velocity jump processes, is presented and analysed.

This modelling approach incorporates recent experimental findings on behaviour of locusts. It exhibits nontrivial dynamics with a "phase change" behaviour and recovers the observed group directional switching. Estimates of the expected switching times, in terms of number of individuals and values of the model coefficients, are obtained using the corresponding Fokker-Planck equation. In the limit of large populations, a system of two kinetic equations with nonlocal and nonlinear right hand side is derived and analyzed. The existence of its solutions is proven and the systemʼs long-time behaviour is investigated. Finally, a first step towards the mean field limit of topological interactions is made by studying the effect of shrinking the interaction radius in the individual-based model when the number of individuals grows. This is a joint work with Radek Erban.

We will give a survey on some recent results on travel tomography which consists in determining the index of refraction of a medium by measuring the travel times of sound waves going through the medium. In differential geometry this is known as the boundary rigidity problem. We will also consider the related problem of tensor tomography which consists in determining a function, a vector field or tensors of higher rank from their integrals along geodesics.

Models for invasions track the front of an expanding wave of population density. They take the form of parabolic partial differential equations and related integral formulations. These models can be used to address questions ranging from the rate of spread of introduced invaders and diseases to the ability of vegetation to shift in response to climate change.

In this talk I will focus on scientific questions that have led to new mathematics and on mathematics that have led to new biological insights. I will investigate the mathematical and empirical basis for multispecies invasions and for accelerating invasion waves.

We review the representation theory of the integrable model underlying the AdS_5/CFT_4 correspondence. We will discuss short and long multiplets, and their impact on the issue of the universal R-matrix. We will give special emphasis to the role of the so-called 'secret symmetry', which completes the Yangian symmetry of the system to a yet to be understood new type of quantum group.

• Rob Style - "Drying and freezing stuff - the wrap up"
• Maria Bruna-Estrach - “Including excluded-volume effects into diffusion of hard spheres" 
• Patricio Farrell - “Multiscale Analysis for Elliptic Boundary Value Problems using Radial Basis Functions"

Would you like to solve a partial differential equation efficiently with a relative error of 10% or would you prefer to wait a bit longer and solve it with an error of only 1% ? Is it sufficient to know that the error is about 1% (having no idea what the `about' means) or would you prefer to have reliable information that the error is guaranteed to be below the required tolerance?

Answering these questions is necessary for the efficient and reliable numerical solution of practically any mathematical problem. In the context of numerical solution of partial differential equations, the crucial tool is the adaptive algorithm with suitable error indicators and estimators. I will overview the adaptive algorithm and its variants. I will concentrate on the a posteriori error estimators with the emphasis on the guaranteed ones.

We define a set of nonlocal operators and develop a nonlocal vector calculus that mimics the classical differential vector calculus. Included are the definitions of nonlocal divergence, gradient, and curl operators and the derivation of nonlocal integral theorems and identities. We indicate how, through certain limiting processes, the nonlocal operators are connected to their differential counterparts. The nonlocal operators are shown to appear in nonlocal models for diffusion and in the nonlocal, spatial derivative free, peridynamics continuum model for solid mechanics. We show, for example, that unlike elliptic partial differential equations, steady state versions of the nonlocal models do not necessary result in the smoothing of data. We also briefly consider finite element methods for nonlocal problems, focusing on solutions containing jump discontinuities; in this setting, nonlocal models can lead to optimally accurate approximations.

This workshop will probably take place at BP's premises.

I describe recent work on the synchronization of IP3R calcium channels in the interior of cells. Hybrid  models of calcium release couple deterministic equations for diffusion and reactions of calcium ions to stochastic gating transitions of channels. I discuss the validity of such models as well as numerical methods.Hybrid models were used to simulate cooperative release events for clusters of channels. I show that for these so-called puffs the mixing assumption for reactants does not hold. Consequently, useful definitions of averaged calcium concentrations in the cluster are not obvious. Effective reaction kinetics can be derived, however, by separating concentrations for self-coupling of channels and coupling to different channels.

Based on the spatial approach, a Markovian model can be inferred, representing well calcium puffs in neuronal cells. I then describe further reduction of the stochastic model and the synchronization arising for small channel numbers. Finally, the effects of calcium binding proteins on duration of release is discussed.

This talk will be based on arxiv:1106.5254.

This talk will be based on arxiv:1106.5254.

This meeting will mark the 80th birthday of Sir Roger Penrose. Twistor theory is one of his most remarkable discoveries and continues to have applications across pure mathematics and mathematical physics. This meeting will focus on some recent developments with speakers both on geometry and physics.

Speakers:

• Nima Arkani-Hamed (IAS, Princeton): Scattering without space-time
• Mike Eastwood (ANU): CR geometry and conformal foliations
• Nigel Hitchin (Oxford): Twistors and Octonions
• Andrew Hodges (Oxford): Polytopes and amplitudes
• Claude LeBrun (SUNY Stony Brook): On Hermitian, Einstein 4-Manifolds
• David Skinner (Perimeter Institute): Scattering amplitudes from holomorphic linking in twistor space
• Paul Tod (Oxford): Conformal cyclic cosmology

Registration will start at 1.30pm on the 21st with the first lecture at 2.15pm. The meeting will finish by 4.30pm on the 22nd. See the programme for more details.

There will be a reception at 6.30pm on the 21st July (Wadham College) followed by dinner at 7.15 in Wadham College.

• Benjamin Franz - "Hybrid modelling of individual movement and collective behaviour"
• Ingrid Von Glehn - "Image Inpainting on Surfaces"
• Rita Schlackow - "Genome-wide analysis of transcription termination regions in fission yeast"

We consider an active scalar equation with singular drift velocity that is motivated by a model for the geodynamo. We show that the non-diffusive equation is ill-posed in the sense of Hadamard in Sobolev spaces. In contrast, the critically diffusive equation is globally well-posed. This work is joint with Vlad Vicol.

There will be a BP workshop but we are waiting for some suggested alternative dates.

Distortion is an asymptotic invariant of the embeddings

of finitely generated algebras. For group embeddings,

it has been introduced by M.Gromov. The main part of

the talk will be based on a recent work with Yu.Bahturin,

where we consider the behavior of distortion functions

for subalgebras of associative and Lie algebras.

Voiculescu showed how the large N limit of the expected value of the trace of a word on n independent hermitian NxN matrices gives a well known von Neumann algebra. In joint work with Guionnet and Shlyakhtenko it was shown that this idea makes sense in the context of very general planar algebras where one works directly in the large N limit. This allowed us to define matrix models with a non-integral  number of random matrices. I will present this work and some of the subsequent work, together with future hopes for the theory.

We present a dynamic bank run model for liquidity risk where a financial institution finances its risky assets by a mixture of short- and long-term debt. The financial institution is exposed to liquidity risk as its short-term creditors have the possibility not to renew their funding at a finite number of rollover dates. Besides, the financial institution can default due to insolvency at any time until maturity. We compute both insolvency and illiquidity default probabilities in this multi-period setting. We show that liquidity risk is increasing in the volatility of the risky assets and in the ratio of the return that can be earned on the outside market over the return for short-term debt promised by the financial institution. Moreover, we study the influence of the capital structure on the illiquidity probability and derive that illiquidity risk is increasing with the ratio of short-term funding.

As with conventional Higgs bundles, calculating Betti numbers of twisted Higgs bundle moduli spaces through Morse theory requires us to

study holomorphic chains. For the case when the base is P^1, we present a recursive method for constructing all the possible stable chains of a given type and degree by representing a family of chains by a quiver. We present the Betti numbers when the twists are O(1) and O(2), the latter of which coincides with the co-Higgs bundles on P^1. We offer some open questions. In doing so, we mention how these numbers have appeared elsewhere recently, namely in calculations of Mozgovoy related to conjectures coming from the physics literature (Chuang-Diaconescu-Pan).

Many radar designs transmit trains of pulses to estimate the Doppler shift from moving targets, in order to distinguish them from the returns from stationary objects (clutter) at the same range. The design of these waveforms is a compromise, because when the radar's pulse repetition frequency (PRF) is high enough to sample the Doppler shift without excessive ambiguity, the range measurements often also become ambiguous. Low-PRF radars are designed to be unambiguous in range, but are highly ambiguous in Doppler. High-PRF radars are, conversely unambiguous in Doppler but highly ambiguous in range. Medium-PRF radars have a moderate degree of ambiguity (say five times) in both range and Doppler and give better overall performance.

The ambiguities mean that multiple PRFs must be used to resolve the ambiguities (using the principle of the Chinese Remainder Theorom). A more serious issue, however, is that each PRF is now 'blind' at certain ranges, where the received signal arrives at the same time as the next pulse is transmitted, and at certain Doppler shifts (target speeds), when the return is 'folded' in Doppler so that it is hidden under the much larger clutter signal.

A practical radar therefore transmits successive bursts of pulses at different PRFs to overcome the 'blindness' and to resolve the ambiguities. Analysing the performance, although quite complex if done in detail, is possible using modern computer models, but the inverse problems of synthesing waveforms with a given performance remains difficult. Even more difficult is the problem of gaining intuitive insights into the likely effect of altering the waveforms. Such insights would be extremely valuable for the design process.

This problem is well known within the radar industry, but it is hoped that by airing it to an audience with a wider range of skills, some new ways of looking at the problem might be found.